This invention generally relates to the field of communications, more particularly, to radio receivers that digitally process a modulated received signal.
With rapid expansion of communication technology and services, demand for digital radio receivers and transmitters have grown exponentially. In radio receivers, Analog-to-Digital (A/D) converters are used to digitize analog radio frequency (RF) signals, which are modulated by a modulating signal. Once digitized, the modulated received signal can be demodulated using well known digital signal processing techniques.
An ideal converter has an error transfer function with a generally sawtooth shape. The error transfer function of such A/D converter has equal incremental steps that represent the differences between input signal levels and corresponding discrete quantized levels. When fabricated using available integrated circuit technologies, however, commercially available A/D converters exhibit inherent non-linearities, which are caused by their internal structure. As a result, the error transfer function of the commercial A/D converters include unequal discrete steps. Furthermore, the inherent non-linearities of a commercial A/D converter may superimpose a shape, for example, an S-shape, on the error transfer function of a commercial A/D converter.
When exposed to inherent non-linearities of the A/D converter, interfering signals produce spurious harmonics, which should be removed to improve receiver performance. For example, if an interfering signal generates spurious harmonics that fall on one or more desired channels, weak received signals on such channels may be masked by stronger spurious harmonics, thus, degrading reception quality on the channels.
The spurious harmonics generated due to non-linearities associated with the internal structure of the A/D converters can not be removed by filtering in the analog domain. In a conventional method, which hereafter is referred to as xe2x80x9charmonic avoidancexe2x80x9d, the harmonics of a large interfering signal are avoided by limiting the received signals to a small part of Nyquist band. However, this method requires sharper (or more narrowband) anti-aliasing filters and severely reduces the usable bandwidth.
Another conventional method uses dithering to eliminate spurious harmonics generated by A/D converters. Dithering is the process of introducing an uncorrelated noise signal, known as dither signal, at the input of an A/D converter. The dither signal smears spurious harmonics over a frequency band. Two well known dithering methods include Subtractive Wideband Dithering and Out-of Band Dithering. Subtractive Wideband Dithering is most effective for generating large dither signals. However, it requires complicated subtracting circuitry for removing the generated dither signal. Dither signals generated by out of band dithering are more easily removed than signals generated by Subtractive Wideband Dithering. For high amplitude signals, however, the advanced analog filter required for generating the dither signals cannot be easily integrated with the circuitry of the receiver.
Dithering effectively removes spurious harmonics, when the amplitude of dither signal is higher than the amplitude of spurious harmonics. Lower order harmonics, such as second and third order harmonics, are created by non-linearities that span a larger part of the A/D converter""s input range than higher order harmonics, such as seventh and eight order harmonics. Therefore, higher order harmonics require lower dithering amplitudes, and are sometimes dithered xe2x80x9cnaturallyxe2x80x9d by present ambient noise. For removing low order harmonics, however, larger dither amplitudes are required. The larger dither amplitudes decrease the dynamic range of the receiver and complicate the design of bandlimiting filters that create and remove dither signals. For low order harmonics of a strong interfering signal that fall on one or more desired channels, however, dithering may itself degrade the reception quality on a channel by reducing the dynamic range and possibly limiting the amplitude range of the A/D converter, which make reception on the channel virtually impossible. Therefore, there exist a need for a method for removing spurious harmonics produced by non-linearities of A/D converters, without the drawbacks associated with dithering.
Briefly, the present invention that addresses this need is exemplified in a radio receiver that avoids spurious harmonics of an interfering signal within the bandwidth of a desired channel using an A/D diversity arrangement.
The radio receiver according to one aspect of the present invention converts an original frequency of a modulated received signal, for example, its second IF frequency, into a plurality of digitization frequencies at a pre-A/D conversion stage. At an A/D conversion stage after the pre-A/D conversion stage, the radio receiver digitizes the modulated received signal at the plurality of digitization frequencies, which are preferably offset from each other, for example, arbitrarily or by a predefined frequency interval, in the order of a few channel spacings. At a post-A/D conversion stage, digitized modulated received signals at each one of the digitization frequencies are converted back to a single common frequency, preferably 0 Hz (baseband). At each one of the digitization frequencies, the radio receiver determines a plurality of reception quality values associated with the digitized modulated received signals, for example, corresponding bit error rates (BER) or carrier-to-interferencexe2x80x94plus-noise ratios (c/(I+N)). A digital signal processor demodulates the modulated received signal by selecting the digitized output of the modulated received signal that was digitized at a digitization frequency that provides a better reception quality than those digitized at another digitization frequency.
According to some of the more detailed features of the invention, the A/D conversion stage digitizes the modulated received signal at the plurality of digitization frequencies substantially simultaneously. In an exemplary embodiment of this arrangement, the A/D conversion stage includes a plurality of A/D conversion branches. The A/D conversion branches include a plurality of frequency converters that are coupled to a corresponding plurality of A/D converters. The frequency converters convert the original frequency of the modulated received signal into the plurality of digitization frequencies during concurrent sampling intervals. The plurality of A/D converters then simultaneously digitize the modulated received signal at the plurality of digitization frequencies during the concurrent sampling intervals.
In an alternative arrangement, the A/D conversion stage digitizes the modulated received signal at the plurality of digitization frequencies substantially non-simultaneously. In an exemplary embodiment of this arrangement, the A/D conversion stage includes a frequency converter, for example, a hopping synthesizer, that converts the original frequency of the modulated received signal into the plurality of digitization frequencies at a corresponding plurality of non-concurrent sampling intervals. An A/D converter non-simultaneously digitizes the modulated received signal at the plurality of digitization frequencies during corresponding plurality of the non-concurrent sampling intervals.
In another more detailed feature of the invention, a dithering circuit introduces a dithering signal at the A/D conversion stage. Preferably, the A/D diversity arrangement of the invention is used to avoid the lower order spurious harmonics, whereas, dithering is used to remove the higher order spurious harmonics.
The radio receiver according to another aspect of the invention determines a frequency location of an interfering signal and calculates the locations of spurious harmonics resulting from digitizing the interfering signal at the plurality of digitization frequencies. The receiver digitizes the modulated received signal at a digitization frequency that resulting spurious harmonics at that frequency do not fall on a desired channel.
According to yet another aspect of the invention, the radio receiver adaptively converts an original frequency of a modulated received signal to a digitization frequency. An A/D converter digitizes the modulated received signal at the digitization frequency. A frequency controller controls the digitization frequency such that spurious harmonics resulting from digitizing an interfering signal at the digitization frequency do not fall on a desired channel.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.